1. Field of the Invention
This invention relates to hearing aids. More specifically it relates to hearing aids having means for altering the spectral distribution of the audio signals to be reproduced by the hearing aid. The invention further relates to methods for processing signals in hearing aids.
Individuals with a degraded auditory perception are in many ways inconvenienced or disadvantaged in life. Provided a residue of perception exists they may, however, benefit from using a hearing aid, i.e. an electronic device adapted for amplifying the ambient sound suitably to offset the hearing deficiency. Usually, the hearing deficiency will be established at various frequencies and the hearing aid will be tailored to provide selective amplification as a function of frequency in order to compensate the hearing loss according to those frequencies.
2. The Prior Art
However, there are individuals with a very profound hearing loss at high frequencies who do not gain any improvement in speech perception by amplification of those frequencies. These steeply sloping hearing losses are also referred to as ski-slope hearing losses due to the very characteristic curve for representing such a loss has in an audiogram. Hearing ability could be close to normal at low frequencies but decreases dramatically at high frequencies. Steeply sloping hearing losses are of the sensorineural type, which is the result of damaged hair cells in the cochlea. Some possible causes of steeply sloping hearing losses are: long-term exposure to loud sound (e.g. noisy work), temporary and very loud sounds (e.g. an explosion or a gunshot), lack of sufficient oxygen supply at birth, various types of hereditary disorder, certain rare virus infections, or possible side effect of certain types of strong medicine. Characteristic signs of steeply sloping hearing loss are the inability to perceive sounds in the high frequencies and a reduced tolerance to loud, high-frequency sounds (sensitivity to sound).
People without acoustic perception in the higher frequencies (typically from between 2-8 kHz and above) have difficulties regarding not only their perception of speech, but also their perception of other useful sounds occurring in a modern society. Sounds of this kind may be alarm sounds, doorbells, ringing telephones, birds singing, or they may be certain traffic sounds, or changes in sounds from machinery demanding immediate attention. For instance, unusual squeaking sounds from a bearing in a washing machine may attract the attention of a person with normal hearing so that measures may be taken in order to get the bearing fixed or replaced before fire or another hazardous condition occurs. A person with a profound high frequency hearing loss, beyond the capabilities of the latest state-of-the-art hearing aid, may let this sound go on completely unnoticed because the main frequency components in the sound lie outside the person's effective auditory range even when aided. No matter how powerful the hearing aid is, the high frequency sounds cannot be perceived by a person with no residual hearing sensation left in the upper frequencies. A method of conveying high frequency information to a person incapable of perceiving acoustic energy in the upper frequencies would thus be useful.
U.S. Pat. No. 5,014,319 proposes a digital hearing aid comprising a frequency analyzer and means for compressing the input frequency band in such a way that the resulting, compressed output frequency band lies within the perceivable frequency range of the hearing aid user. The purpose of this system, known as digital frequency transposition (DFC), is to enhance phonemes with significant high frequency content, especially plosives and diphthongs, in speech by compressing the upper frequency band in such a manner that the frequencies where the plosives and diphthongs occur are moved sufficiently downward in frequency to allow them to be perceived by a hearing impaired hearing aid user. The system is dependent on the characteristics in the incoming signal and the frequency analyzer in order to function properly. Other sounds in the upper frequency band are not detected by the frequency analyzer, and their frequencies are therefore not compressed and thus remain undetectable by the user. The frequency analyzer has to be very sensitive in order for phonemes to be correctly recognized. This puts a great strain on the hearing aid signal processor.
EP 1 441 562 A2 discloses a method for frequency transposition in a hearing aid. A frequency transposition is applied to the spectrum of a signal, using a nonlinear frequency transposition function so that all frequencies above a selected frequency fG are compressed in a nonlinear manner and all frequencies below the selected frequency fG are compressed in a linear manner. Although the lower frequencies are compressed in a linear manner in order to avoid transposition artifacts, the whole useable audio spectrum is nonetheless compressed, and this may lead to unwanted side effects and an unnaturally sounding reproduction. The method is also very processor intensive, involving FFT-transformation of the signal to and from the frequency domain.
U.S. Pat. No. 6,408,273 B1 discloses a method for providing auditory correction for hearing impaired individuals by extracting pitch, voicing, energy and spectrum characteristics of an input speech signal, modifying the pitch, voicing, energy and spectrum characteristics independently of each other, and presenting the modified speech signal to the hearing impaired individual. This method is elaborate and cumbersome, and appears to affect the sound image in a negative way because the entire perceivable frequency spectrum is processed. This kind of intensive processing inevitably distorts the overall sound image, perhaps even beyond recognition, and thus presents the user with perceivable, but unrecognizable, sound.
The methods of frequency transposition known in the prior art all affect the low frequency content of the processed signal in some form. Although these methods render high frequency components in the signal audible to persons with steep hearing losses, they also compromise the integrity of the overall signal, making a lot of well-known sounds hard to recognize with this system. In particular, the amplitude-modulated envelope of the input signal is deteriorated badly with any of the known methods. An effective, fast and reliable method for making high frequency sounds available to hearing impaired people, without compromising the quality of the result significantly, is thus desirable.